Sir Stephen Lamport, his former private secretary, said the Prince believes that writing to Government ministers prepares him for when he is made king and will stop the controversial practice when he is crowned.

Sir Stephen's comments were made in a series of evidence sessions to a tribunal of judges which ruled on Tuesday that the Government should release some of the Prince's letters under the Freedom of Information Act.

The Prince has faced criticism for sending the letters - known as “black spider memos” because of his handwriting - to ministers, marked private and confidential, setting out his views on aspects of government policy.

Sir Stephen’s comments emerged in a 126-page Freedom of Information ruling that departments should disclose contents of letters from Prince of Wales seven years ago. The law has since changed so more recent letters are unlikely to be released.

According to the ruling, Sir Stephen stressed that the “public perception” of the prince’s party political neutrality was “of crucial importance” and that “public knowledge” of his letters could damage it, particularly after he became king.

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It said: “Sir Stephen’s view was that public knowledge of his letters and their contents would give rise to a different public perception of his neutrality when he became sovereign and ‘if you inhibit and corrode that now I don’t know how you restore it’.”

This was because the Prince believes that the “cardinal principle” of never commenting on public policy did not apply to him as heir, only to the Monarch, it said.

Sir Stephen told the judges that the Prince did not consider that "the cardinal principle that he shouldn’t express views publicly on matters of public policy" applied to him. This was a principle that would apply to the sovereign.

Sir Stephen said "was certain that Prince Charles’ ability to discuss contentious matters with ministers is part of his preparation for kingship”, the judgement said.

Sir Stephen “expressed the view that whilst this may not be ‘a right’ it is part of the constitutional convention and ‘everyone seems happy with it’”, it added.

Sir Stephen felt that the prince, according to the judgement, “needs to prepare for being king by being briefed about the nation’s affairs and by expressing his own views to Government… and that if what Prince Charles says or writes to ministers were not to be confidential their exchanges would be ‘bland and denuded’ of any useful content.”

Sir Stephen, who worked for the Prince as deputy private secretary and then private secretary from 1993 to 2002, said that the Prince considered his letters to ministers to be preparation to be king under the “apprenticeship convention”.

According to the summary of Sir Stephen’s evidence, the convention covered “all his interactions with ministers [and] formed part of building a ‘tapestry of relations’ which were an essential part of his preparations to be sovereign”.

This convention included the “urging of opinions” to ministers by the Prince of Wales, who viewed this as part of his role as heir to the throne.

Paul Richards, a former Labour special adviser between 2005 and 2009, told the tribunal, that the Prince’s letters were “put before the minister, effectively at the top of the file and treated with great reverence.

“He gave a number of examples. This contrasts with ordinary citizens whose letters go through a centralised mailroom and which are normally dealt with by departmental staff and rarely seen by ministers or their advisers.”

Three judges ruled that “it was fundamental” that the Prince’s letters “cannot have constitutional status” and cannot be protected from disclosure.

As a result of the ruling, seven Whitehall departments will now have to hand over within a month letters sent during a seven-month period in 2004 and 2005, unless they lodge an appeal at the court of appeal.

In June 2001 the Prince complained to the then Lord Chancellor Lord Irvine, that the UK was “sliding inexorably down the slope of ever increasing, petty-minded litigiousness”.

In the following year he wrote again to Lord Irvine worrying about “ever-more prescriptive laws - for example, heath and safety at work legislation, the blame culture… and the bureaucratic red tape which accompanies new rules”.